1. Field of the Invention
This invention relates to a rubber composition for a tire tread, and more particularly to a rubber composition suitable for use in a tread of a pneumatic tire considerably reducing rolling resistance without degrading wet-skid resistance, fracture resistance and wear resistance.
2. Description of the Related Art
Recently, it becomes important to study the reduction of the rolling resistance in the tire for saving the fuel consumption in the automobile in accordance with social demands of resource-saving and energy-saving. In general, it is known that the fuel consumption of the automobile is mitigated by reducing the rolling resistance of the tire or by so-called low fuel consumption tire. In order to reduce the rolling resistance, it is common to use a rubber material having a small hysteresis loss as a tread rubber of the tire. On the other hand, it is strongly demanded to use a rubber material having a high frictional resistance on wet road surface (wet skid resistance) from a viewpoint of the demand on the running stability.
However, the low rolling resistance and the high frictional resistance on wet road surface are conflicting with each other, so that it is very difficult to simultaneously satisfy these properties.
Lately, the rolling resistance and wet skid resistance of the tire are theoretically related to viscoelastic properties of the rubber composition. In this case, it is effective to decrease the hysteresis loss of the tread rubber for reducing the rolling resistance during the running of the tire, or viscoelastically decrease the loss tangent (tan .delta.) at a temperature of 50-70.degree. C. corresponding to the running tire temperature in order to obtain a low fuel consumption. On the other hand, it is known that the wet skid resistance is well interrelated to the loss tangent at about 0.degree. C. under a frequency of 10-20 Hz, so that it is necessary to increase the loss tangent at about 0.degree. C. in order to improve the gripping property of the tire.
As a method of decreasing the hysteresis loss or tan .delta. at a temperature of 50-70.degree. C., it is general to use a rubber material having a low glass transition temperature (Tg) such as high-cis polybutadiene rubber or a rubber material having a high rebound resilience such as natural rubber.
However, the wet skid resistance is extremely lowered in these rubber materials, so that it is considerably difficult to simultaneously establish the running stability and the low rolling resistance according to the conventional techniques.